Integrating self-organizing mapping and causal inference to assess patterns and drivers of surface river water quality in a coastal province of China.

River water quality degradation is a prevailing problem in coastal China with intensifying human-nature interaction. However, the spatial and temporal dynamics of water quality and their drivers remain poorly understood. In this study, we developed an analytical framework integrating self-organizing mapping (SOM) with partial least squares structural equation models (PLS-SEMs) to analyze the patterns and drivers of river water quality at 49 stations from 2021 to 2023 in Fujian Province, a coastal region in southeastern China. Two regions with distinctive water quality characteristics were divided and then seasonal patterns for six water quality parameters in these two regions were delineated using SOM. Multiple PLS-SEMs across the whole province and subregions were then developed to identify the key drivers of water quality dynamics. The overall performance of PLS-SEMs was satisfactory, most latent variables had average variance extracted (AVEs) higher than 0.5 and factor loading estimates greater than 0.7. The environmental latent variables including temperature, elevation, and rainfall exert a significant negative effect, with a path coefficient of -0.509, indicating an improving effect on water quality in whole province. Anthropogenic latent variables including built-up and farmland had a stronger deteriorating impact on water quality than environmental latent variables in estuaries of Fujian Province, with a path coefficient of 0.933. For coastal areas, the anthropogenic latent variables had a deteriorating impact on river water quality (path coefficient = 0.644). Our results revealed that environmental factors dominated water quality dynamics at the provincial scale, while anthropogenic factors mainly drove variations in water quality in estuarine areas. This study might provide promising insights into sustainable water management in Fujian province and other similar coastal regions.